Many types of fuel cells are known in the art, such as solid oxide fuel cells, molten carbonate fuel cells, phosphoric acid fuel cells and proton exchange membrane (PEM) fuel cells. Conceptually, the operation of a fuel cell is very simple. An electrolyte separates an anode and a cathode, between which electricity is produced when a fuel is introduced to the anode, an oxidizer is introduced to the cathode, and the cell is maintained within a proper temperature range. The electrolyte allows an ionic species to travel between the cathode and the anode. The reaction products are relatively simple and benign, typically including water and carbon dioxide, thus minimizing environmental concerns, and the operating efficiencies are theoretically relatively high. In contrast to other energy sources, such as internal combustion engines, fuel cells are simpler, less noisy, do not pollute, and create electricity directly. Accordingly, fuel cells are considered by many to hold considerable promise as the power source of the future,
In reality, however, fuel cell systems can be relatively complex, as considerable hardware can be required to support the fuel cells. Such hardware can include thermal management subsystems, fuel processing subsystems, and water management sub-systems. Furthermore, in practice, particular detailed procedures for changing the state of operation of the fuel cell, such as shutting the cell down, or starting the cell up, are often required to avoid damage to the fuel cell.
For example, it can be necessary or desirable to shut down the fuel cell for any number of reasons. An emergency shutdown can be required because a monitored parameter is out of an acceptable range; a shut down can be scheduled for maintenance of the cell or of the associated subsystems, or servicing of the load may necessitate a "hot hold." Each of these state changes typically involves immediately removing the electrical load from the fuel cell.
Such state changes can damage the fuel cell if electrode potentials, which typically rise upon removal of the load, are not properly controlled when the cell is at an elevated temperature. For example, as disclosed in U.S. Pat. No. 5,045,414, issued to Bushnell et al. and herein incorporated by reference, in a phosphoric acid fuel cell the cathode can undergo catalyst dissolution, catalyst support corrosion and catalyst layer flooding, if the potential exceeds eight tenths (0.8) of a volt. On the other hand, if the cathode potential approaches the anode potential and the cathode is subsequently re-oxidized, the catalyst can recrystallize and lose activity. If the anode reaches the potential of the cathode, it can flood with electrolyte. Accordingly, the control of electrode potentials is an important concern.
The '414 patent discloses a variation of a technique, known in the art, of purging and passivating the fuel cell upon removal of the service load. According to the '414 patent, nitrogen is added to the cathode flow field to create a nitrogen/oxygen mix having less than 1% oxygen by volume, and nitrogen is supplied to the anode. The concentration of oxygen supplied to the cathode is important, as low a concentration can prevent achieving the proper potential and too high a concentration can cause catalyst dissolution, corrosion of the catalyst support, and catalyst layer flooding. The proper concentration of oxygen can depend on several factors, including the matrix thickness and composition of the electrolyte, as well as the temperature of the fuel cell. A dummy load is often switched to the cell to bring the cathode potential down rapidly at the start of the purges.
However, such approaches are not entirely satisfactory. The possibility of damage to electrodes remains due to the relatively sudden and large change in electrical loading on the cell. Furthermore, reconnecting the service load after a "hot hold" requires purging the system of the inert gases and reintroducing the reactants at the appropriate rates and concentrations to achieve damage-free normal power operation.
As a simpler and more effective technique for changing the state of operation of a fuel cell would be a welcome advance, it is an object of the invention to improve upon the aforementioned methods and apparatus of the prior art, and to address one or more of the disadvantages and drawbacks thereof.
It is another object of the invention to provide improved methods and apparatus for changing the state of operation of a fuel cell.
Other objects of the invention will in part be apparent and in part appear hereinafter.